1. Field of the Invention
The present invention relates to an aqueous resin composition used to form an undercoat layer of a thermosensitive recording medium having excellent performance. More particularly, the present invention relates to an aqueous resin composition used to form an undercoat layer of a thermosensitive recording medium, which has high dynamic color developing sensitivity and excellent head-matching properties and is capable of recording uniform and clear images with high density, by providing an intermediate layer (hereinafter referred to as an “undercoat layer”) between a support made of a paper or polyethylene terephthalate (PET) and a thermosensitive color developing layer, and to a thermosensitive recording medium.
2. Description of the Related Art
With rapid increase in amount of information, variation of kinds, resource conservation, and social demands such as environmental response, various recording materials have been put into practical use in the field of information recording. Among these, a thermosensitive recording medium is widely used in the fields of printers (computer output, desk-top calculators, etc.), recorders for medical measurement, low-speed and high-speed facsimiles, automatic ticket vendors, thermal copying machines and labels for POS (electronic cash register) systems because of the following advantages:
(1) complicated developing processes of the prior art are not required because color-developed images are recorded only by heating,
(2) recording can be conducted by using relatively simple and compact apparatus and the thermosensitive recording medium is easily handled and requires low maintenance costs,
(3) cost is low because a paper can be used as a support, and
(4) feel of the resulting recording material resembles that of plain paper.
The above thermosensitive recording medium is commonly produced by applying a thermosensitive color developing resin composition containing color developing component capable of causing the color development reaction due to heat on a support made of a paper, a synthetic paper or a synthetic resin film and drying the thermosensitive color developing resin composition, and color-developed images are recorded by heating the thermosensitive recording medium thus obtained using a thermal recording device such as a heat stylus or a thermal head.
A conventional thermosensitive recording medium had a problem in that color developing sensitivity, namely, “dynamic color developing sensitivity” is insufficient when a thermal head is driven at high speed.
To solve such a problem, for example, Japanese Patent Application, First Publication No. Hei 1-113282 A proposes a method of using plastic spherical hollow particles having an average particle diameter of 0.2 to 1.5 μm and a hollow percentage of 40 to 90% in an undercoat layer. However, this method was not a satisfactory method because of poor flexibility of the wall material of hollow particles or poor thermal insulation properties due to poor expandability, and poor approachability between the thermal head and the thermosensitive recording medium.
Also Japanese Patent Application, First Publication No. Hei 4-241987 A proposes a method of using a fine hollow filler having a volumetric hollow ratio of 90% or more in an undercoat layer. However, there was the following problem. As the hollow ratio of the hollow filler increases, thermal insulation properties are enhanced and the sensitivity is improved; however, the specific gravity of the filler itself is reduced. When a composition (coating solution) for an undercoat layer is made using the filler, concentration gradient of the hollow filler occurs at the portion over the liquid and the portion under the liquid over time, and thus the hollow filler on the liquid level is dried, bonded and coagulated.
Furthermore, the thermosensitive recording medium had a problem in that homogeneous images were difficult to obtain because the surface of the undercoat layer has poor smoothness.
As a means for solving the problem, various proposals are made in Japanese Patent Application, First Publication No. Sho 59-225987 A and Japanese Patent No. 2630945. For example, a technique of laminating a foamed layer obtained by foaming a foamable plastic filler with an undercoat layer containing a filler and a binder and a technique of adding inorganic and organic fillers to a foamable plastic filler are proposed. The former technique has a problem such as a complicated manufacturing process while the latter technique has a problem such as increased cost with an increase in kinds of raw materials, and thus there have never been obtained results which meet all demands for quality, cost, and manufacturing process of the thermosensitive recording medium.
The foamable plastic filler used in the latter technique is a hollow filler comprising a shell made of a thermoplastic substance and a low-boiling point organic solvent such as propane or isobutane contained in the shell, and evolves a carbon dioxide gas or an organic gas into an air when foamed by heating. This fact goes against the trend of recent reduction of the environmental burden and is one of problems which should be considered with respect to a means for reducing the environmental burden of raw materials used in the undercoat layer.